Book excerpt: In this book, a new approach to the theory and practice of two-phase systems based on a global invariant – entropy, – and other invariants is formulated and experimentally confirmed. - Offers a novel approach to the study of the two-phase flows systems based on statistical mechanics and probability theory - Provides the tools for computing and modelling two-phase systems, predicts mass transfer and enables system optimization - Provides a plethora of examples in among others, separation processes, dust production, pneumatic transport, and boiling bed